After breathing in a test tube that contains acidified K\(_2\)Cr\(_2\)O\(_7\), a man noticed a change in the colour of the acidified K\(_2\)Cr\(_2\)O\(_7\) ...

Acidified potassium dichromate (K₂Cr₂O₇) is a strong oxidising agent. When it oxidises a substance, the orange dichromate ion (Cr₂O₇^2-, containing Cr⁶⁺) is reduced to the green chromium(III) ion (Cr³⁺). This colour change from orange to green is the characteristic indicator that an oxidation reaction has occurred.

Human breath can contain ethanol (an alkanol) if the person has consumed alcohol. Ethanol is readily oxidised by acidified dichromate:

\[\text{C}_2\text{H}_5\text{OH} + [\text{O}] \rightarrow \text{CH}_3\text{CHO} + \text{H}_2\text{O}\]

and further:

\[\text{CH}_3\text{CHO} + [\text{O}] \rightarrow \text{CH}_3\text{COOH}\]

This is the principle behind the breathalyser test used to detect alcohol (alkanol) in a person's breath. The colour change from orange to green confirms the presence of an alkanol (alcohol).

Alkanones (ketones) are not easily oxidised by acidified dichromate under mild conditions, so they would not produce this colour change. Alkanals (aldehydes) and alkanoic acids (carboxylic acids) are not typically present in human breath in significant amounts.